Treatment of planarizing layer in multilayer electron beam resist

ABSTRACT

An improved electron beam resist structure comprises an organic planarizing layer which has been treated with an ion beam for a time sufficient to render it conductive and an electron beam resist layer thereover. The electron beam resist layer is preferably oxygen plasma resistant. When the resist layer is not resistant to oxygen plasma and it is desired to develop the planarizing layer by oxygen plasma, the structure additionally includes a thin hard mask layer, suitably of silicon dioxide, interposed between the conductive planarizing layer and the resist layer.

This invention relates to the formation of a patterned structure on asubstrate useful for electron beam lithographic processing thereof.

BACKGROUND OF THE INVENTION

The use of electron beam irradiation to form a patterned resiststructure on a substrate is known. The use of such resist structures forthe further processing of the substrate, e.g. selective etching, metaldeposition and the like, is likewise known. The increasing demand forvery-large-scale-integrated (VLSI) devices has made imperative theformation of such structures having patterns on the order of onemicrometer and below with accuracy and reproducibility.

The capability of electron beam generating equipment to produce anexceptionally narrow beam, e.g. 0.5 micrometer and below, is asignificant advantage for fine-line lithography. It is recognized,however, that electron beam irradiation will backscatter from thesubstrate surface in much the same manner as light utilized to irradiatea photoresist will reflect from the substrate with a resultant loss ofpattern resolution.

A second problem inherent in electron beam lithography is the build-upof a charge in the resist layer and on the substrate surface duringirradiation. The deposited charge can cause aiming errors and patternmisalignment, both in orientation on the substrate surface and in theactual writing of the pattern.

The backscattering of an electron beam, commonly referred to as theproximity effect, has been addressed, for example, by varying the localirradiation, i.e. the scan rate and the dwell time, of the beam. Whilethis is a reasonable approach for low-powered apparatus, variance oflocal irradiation is, at best, limited as the capacity of such apparatusapproaches 300 MHz.

An approach to solving the problem of charge build-up is the depositionof a thin conductive coating on the substrate, or on the resist itself,as described in U.S. Pat. No. 3,893,127, issued July 1, 1975. This film,which is preferably 10-100 nm thick, is of a conductive metal, e.g.copper, aluminum, nickel and the like, or a layer of glass with a thincoating of a conductive oxide such as tin oxide or indium oxide. The useof such coatings is disadvantageous in that they require extra processsteps for their patterning and subsequent removal.

U.S. Pat. No. 4,456,677, issued June 26, 1984, discloses a compositeresist structure wherein a high atomic number film, e.g. gold, overliesa low atomic number film, e.g. aluminum, which in turn overlies a layerof positive resist. The thickness of the two films is selected so thatonly a small percentage of the electrons striking the top layer actuallypasses into the resist film. It is also stated that a single layer ofresist impregnated with a high atomic number material could be utilized.In either instance, the high atomic number material will absorb asubstantial portion of the electron energy, thus requiring the use of ahighly sensitive resist material.

Another proposed electron beam resist medium is a trilayer structurecomprising a base or planarizing layer and an electron beam resist layerseparated by a thin layer, e.g. 60-100 nm thick, of silicon. In such astructure, the bottom layer must be sufficiently thick to preventproximity effect and the silicon layer sufficiently thin so that it doesnot cause backscattering of the electron beam, yet sufficiently thick todissipate charge build-up. It will be appreciated that, in addition tothe extra process steps required for a three layer structure, both ofthe structures described above require that at least one of the layersmeet very exacting criteria for thickness and uniformity.

A simplified resist structure for electron beam lithography is providedin accordance with this invention.

SUMMARY OF THE INVENTION

An electron beam resist structure is formed by spin-coating a substratewith a layer of a suitable polymer, treating the polymer layer with anion beam to render it conductive and depositing a layer of a suitableelectron beam resist thereover.

DETAILED DESCRIPTION OF THE INVENTION

The substrate upon which a patterned electron beam resist structure isformed in accordance with this invention may be any suitable material,for example, single crystalline silicon, gallium arsenide, tungsten,polycrystalline materials with suitable surfaces, vitreous materials orthe like. The substrate may be appropriately doped to provide regions ofvarying conductivity. Topographical features on the substrate, ifpresent, typically are devices, device subassemblies, silicon islands,mesas, circuit components and the like, and may be of the same or amaterial different from that of the substrate. The exact nature of suchfeatures is not critical to the invention.

A layer of a suitable organic polymeric material is initially depositedover the substrate. This layer will be referred to herein as aplanarizing layer even if it is applied over a substrate having notopographical features. The polymer material, in general, must adherewell to the substrate, be suitable for the ion beam treatment describedhereinafter, be otherwise inert and, ideally, have good planarizingcapability. Particularly suitable materials include certain polyimides,and certain positive photoresists such as the novolac resin/diazoquinonesensitizer compositions. Our experience has shown that some organicplanarizing materials, for example, poly(methylmethacrylate) aresignificantly eroded during the ion beam treatment contemplated herein.Such materials are not useful in the present process. The organicplanarizing layer is suitably deposited onto the substrate byspin-coating and then cured by heating to the appropriate temperature.Polyimide layers, for example, are heated to from about 350°-400° C.,suitably in an inert atmosphere, for from about 20 to 60 minutes.

The thickness of the planarizing layer must be sufficient to preventelectrons backscattering from the substrate from reaching the resistlayer. In the event that the substrate has topographical features, thethickness of the planarizing layer is suitably from about 1 to 3 timesthe height of the highest such feature. Preferably, the thickness of theplanarizing layer is at least equal to the height of the highesttopographical feature on the substrate. In general, the thickness of theplanarizing layer is at least one micrometer, and suitably between aboutone and three micrometers. The minimum thickness of the planarizinglayer will depend, in part, on factors such as the thickness andcomposition of the resist layer, the contemplated intensity and energyof the electron beam irradiation and the like.

In accordance with this invention, the planarizing layer is ion beamtreated for a time sufficient to render it conductive. Although theexact mechanism responsible for the conductivity is not known withcertainty, it is believed that the ion beam "graphitizes" or"carbonizes" the surface. The ion beam may also be severing bonds in thepolymer lattice thereby creating free electrons. The ion beam treatmentmay be carried out with ions generally recognized as being electricallyinert, such as argon, helium or nitrogen, or with electrically activeions such as arsenic, phosphorus or boron. Inert ions are particularlysuitable in that they produce a minimum of side reactions. Theparameters of the treatment will depend on the ion being utilized.Argon, for example, will produce a conductive surface on a polyimidelayer at a plate bias of 500V, 1×10⁻ 5 torr and a beam current of3ma/cm². The ion treatment is carried out using conventional apparatusand conditions.

The ion beam treatment of the planarizing layer significantly increasesthe conductivity thereof. For example, an untreated layer of aconventional polyimide planarizing layer 2 micrometers thick has aresistance in excess of about 100 milliohms. Ion beam treatment of thelayer with argon ions for 5 minutes as described above reduced theresistance, i.e. increased conductivity, to about one milliohm. Thislevel of conductivity is more than sufficient to dissipate the chargebuild-up in conventional electron beam pattern irradiation. In general,the resistivity of the conductive planarizing layer after ion beamtreatment should be at least about 10 milliohms.

The conductive ion beam treated planarization layer is coated with anelectron beam resist. Particularly suitable electron beam resists arethose which are resistant to oxygen plasma. Typical of such resists arepoly(silane sulfone) copolymers disclosed in U.S. Pat. No. 4,357,369,issued Nov. 2, 1982 and represented by the formula ##STR1## wherein R ateach occurance is an alkyl group, suitably lower alkyl group, and n isan integer. Such copolymers suitably have a molecular weight of fromabout 50,000 to about 200,000 or above. Other conventional electron beamresists may be utilized as well. These include, for example,poly(1-butene sulfone) and COP, a copolymer of ethylacrylate andglycidyl methacrylate, both available from Mead Chemical Co.

The resist layer is applied to the planarizing layer, suitably byspin-coating, and dried to form a layer from about 0.25 to 1.0micrometer thick. The resist layer is suitably soft-baked at therecommended temperature to remove residual solvent.

In the event that the electron beam resist utilized to coat theplanarizing layer does not have resistance to an oxygen plasma etch, itcan be made resistant by treatment with, e.g., titanium tetrachloride orhexamethylene disilazane. It is also within the scope of this inventionto utilize a thin hard-mask layer of a nonconductive material whichwould give minimal backscattering, preferably silicon dioxide. Asuitable layer of silicon dioxide would be deposited over theplanarizing layer by a conventional technique, such as evaporation,suitably to a thickness of from about 20 to 200 nanometers. Neither theapplication of a hard-mask layer, when present, nor application of theresist layer causes any significant loss in the conductivity of theplanarizing layer.

The resist layer is pattern irradiated with an electron beam or amodulated electron beam, and developed with a suitable developer.Silicon-containing oxygen etch-resistant resist compositions such asthose in the above-mentioned patent are developed with an organicdeveloper such as, for example, alcohols such as 2-methoxyethanol,isopropanol, ethoxyethanol and the like. These alcohols may be utilizedindividually, in combination, or in combination with a ketone developersuch as, for example, acetone, ethylacetoacetate, tetrahydrofuran andthe like.

The exposed portion of an intermediate hardmask layer, if present, isremoved by conventional wet or dry etching. Silicon dioxide, forexample, is suitably wet etched with buffered hydrofluoric acid, or dryetched in a fluorocarbon plasma. Since the hardmask layer is very thin,it is generally possible to achieve an anisotropic etch by carefulcontrol of the etch conditions.

The exposed portion of the conductive planarizing layer is then removed,suitably in an oxygen plasma. The ion beam treatment of the planarizinglayer has no effect on its development. It is also possible to wetdevelop the planarizing layer, if the developer has substantially noeffect on the resist layer. For fine-line electron beam lithography,however, dry development is preferred.

The resist structure provided in accordance with this invention isadvantageous in that it has substantially reduced proximity effect fromelectron beam irradiation. It also effectively dissipates charge buildupby electron beam irradiation without the use of a metallic layer whichmight enhance the proximity effect of the irradiation. The subjectresist structures additionally provide electron beam pattern delineationat the one micrometer level and below with relative ease of processing,particularly in comparison to systems having multiple layers of metal.

The following Example further illustrates this invention, it beingunderstood that the invention is in no way intended to be limited to thedetails described therein. In the Example, all parts and percentages areon a weight basis and all temperatures are in degrees Celsius, unlessotherwise stated.

EXAMPLE

The substrates for this example were three inch silicon wafers havingepitaxial silicon islands 120 micrometers square and 0.5 micrometer inheight. Dupont PI2555, a polyimide preparation, was spin-coated ontothree substrates and cured at 400° under a nitrogen ambient for 30minutes to form a polyimide layer 2.0 micrometers thick.

Utilizing an ion beam bombardment system, the polyimide layer on eachwafer was treated for 2 minutes at ambient temperature with argon ionsat a 3 milliamp/cm² current density and a plate voltage of 500 V. Theelectrical resistance of the surface of each wafer was about onemilliohm.

HPR-204, Hunt Chemical Company, a commercial photoresist having e-beamsensitivity was spin-coated onto the wafers to a thickness of 1.0micrometer. The films were allowed to dry and then baked at 95° forthirty minutes to remove residual solvent.

The wafers were irradiated in a scanning electron beam microscope whichproduced a beam approximately 0.5 micrometer in diameter. A pattern oflines and spaces 1.0, 1.5, 2.0 and 2.5 micrometers in width was formedon the resist layer of each wafer.

The pattern was developed with the developer recommended by themanufacturer in each instance. The resist layer showed excellent patterndelineation with no apparent erosion. There was no loss of resolutionfrom proximity effects or charge buildup.

We claim:
 1. A process of forming a patterned electron beam resiststructure on a substrate comprising:(a) coating the substrate with alayer of organic planarizing material; (b) treating the planarizinglayer with an ion beam for a time sufficient to render it conductive;(c) coating the conductive planarizing layer with a layer of electronbeam resist; (d) pattern irradiating the resist layer with an electronbeam; (e) developing the resist layer; and (f) developing the underlyingportion of the planarizing layer thereby exposing a portion of thesubstrate.
 2. A process in accordance with claim 1, wherein the resistlayer is resistant to oxygen plasma etching and the planarizing layer isdeveloped in an oxygen plasma.
 3. A process in accordance with claim 2,wherein the resist is a poly(silane sulfone) copolymer represented bythe formula ##STR2## wherein R at each occurrence is an alkyl group andn is an integer.
 4. A process in accordance with claim 1, wherein theplanarizing layer is a polyimide.
 5. A process in accordance with claim1, wherein the planarizing layer is treated with argon, helium ornitrogen ions.
 6. A process in accordance with claim 5, wherein theplanarizing layer is treated with argon ions.
 7. A process in accordancewith claim 1, additionally including the steps of depositing a thinhardmask layer over the conductive planarizing layer and developing theportion of said hardmask layer exposed by development of the resistlayer.
 8. A process in accordance with claim 7, wherein the hardmasklayer is silicon dioxide.
 9. A process in accordance with claim 8,wherein the hardmask layer is developed in a fluorocarbon plasma and theunderlying portion of the planarizing layer is developed in an oxygenplasma.